KR20100029811A - Organic light emitting display apparatus and driving method thereof - Google Patents

Abstract

PURPOSE: An organic light emitting diode and a driving method thereof are provided to obtain a uniform brightness by preventing a shift of a threshold voltage and the degradation of a first transistor. CONSTITUTION: A pixel circuit of an organic light emitting diode display device includes a OLED, a first transistor(Tr1), a second transistor(Tr2), a capacitor(C), and a third transistor(Tr3). The anode of the OLED is connected to a second electrode of the first transistor. A ground voltage is applied to the cathode of the OLED. Power voltage is applied to a first electrode of the first transistor and a data signal is applied to a gate electrode. A second electrode of the first transistor is electrically connected to the OLED.

Description

Organic light emitting display and driving method thereof

The present invention relates to an organic light emitting display device and a driving method thereof, and more particularly, to an organic light emitting display device and a driving method thereof for maintaining a threshold voltage of a driving transistor.

In general, an organic light emitting display device is a display device for electrically exciting a fluorescent organic compound to emit light, and is capable of displaying an image by voltage driving or current driving a plurality of organic light emitting cells arranged in a matrix form. These organic light emitting cells have diode characteristics and are called organic light emitting diodes (OLEDs).

1 is a conceptual diagram of an organic light emitting device.

Referring to FIG. 1, the organic light emitting diode has a structure of an anode (ITO), an organic thin film, and a cathode electrode layer (metal). The organic thin film has a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), and an electron transport layer (electron) to improve the emission efficiency by improving the balance between electrons and holes. transport layer (ETL) and electron injection layer (EIL).

The organic light emitting device may be driven by a simple matrix method and an active matrix method using a thin film transistor (TFT) or a MOSFET. In the simple matrix method, the anode and the cathode are orthogonal and the line is selected and driven, whereas the active driving method connects a thin film transistor to each indium tin oxide (ITO) pixel electrode and is maintained by a capacitor capacity connected to the gate of the thin film transistor. It is driven according to the voltage. On the other hand, the active driving method is divided into a voltage programming method and a current programming method according to the type of signal applied to write and maintain a voltage in the capacitor.

2 is a circuit diagram of a pixel employed in a conventional organic light emitting display device.

Referring to FIG. 2, a pixel of an organic light emitting diode display includes an organic light emitting diode OLED, two transistors M1 and M2, and one capacitor C _st , and generally the switching transistor M1 and the driving device. As the transistor M2, a thin film transistor TFT is used.

In the pixel circuit of FIG. 2, the first electrode of the switching transistor M1 is connected to the data signal line. At this time, the switching transistor M1 is turned on by the scan signal S [n] applied to the gate electrode, so that the data signal D [m] is applied to the pixel circuit.

The capacitor C _st is connected between the first electrode and the gate electrode of the driving transistor M2 to maintain a data voltage corresponding to the data signal for a predetermined period of time. In addition, the driving transistor M2 supplies a current corresponding to the voltage applied between both electrodes of the capacitor C _st to the organic light emitting diode OLED.

It said switching transistor (M1) is the ground is turned on, the data voltage applied through the data signal line is stored in the capacitor (C _st), since stored in the switching transistor and the capacitor (C _st) in the case that (M1) is turned off data A current corresponding to the voltage is applied to the organic light emitting diode OLED through the driving transistor M2. Accordingly, the organic light emitting diode OLED maintains light emission for a predetermined period even when the switching transistor M1 is turned off.

At this time, the current flowing through the OLED is represented by Equation 1 below.

In Equation 1, I _OLED is a current flowing through the OLED, V _gs is a voltage between the gate electrode and the first electrode of the driving transistor M2, V _th is a threshold voltage of the driving transistor M2, V _DD represents a power supply voltage, V _data represents a data voltage, and β represents a gain factor.

However, if a positive or negative voltage is continuously applied to the gator electrode of the driving transistor M2, the V _th value is shifted by the voltage stress. Therefore, when a circuit having a general two transistor one capacitor structure is used, deterioration of the driving transistor M2 occurs, and after a few hours, the V _th value is shifted to give a luminance different from the first. Therefore, it is necessary to prevent the phenomenon in which the V _th value of the driving transistor M2 is shifted.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide an organic light emitting display device and a driving method thereof for preventing deterioration of a driving transistor provided in a pixel circuit.

In order to solve the above technical problem, the first side of the present invention includes an organic light emitting device for generating light, a first electrode to which a power supply voltage is applied, and a second electrode and a gate electrode electrically connected to the organic light emitting device. A second transistor having a first transistor, a first electrode to which a data signal is applied, a second electrode connected to the gate electrode of the first transistor, and a gate electrode to which a first scan signal is applied, the second transistor of the first transistor A capacitor having a first terminal connected to a gate electrode and a second terminal connected to the first electrode of the first transistor, and a first electrode connected to a gate electrode of the first transistor and a second voltage applied with a ground voltage And a third transistor having an electrode and a gate electrode to which the second scan signal is applied, wherein the second scan signal is a phase. The provides an organic light emitting diode display, it characterized in that the scanning signal applied to a pixel adjacent to the right before applying the first scanning signal.

The organic light emitting diode display may include a pixel area including a pixel circuit and a non-pixel area disposed outside the pixel area and including a driving circuit driving the pixel circuit.

In the present invention, the third transistor may be included in either the pixel region or the non-pixel region.

In the present invention, when the third transistor is included in the pixel region, the organic light emitting diode display may have a top emission structure.

In the present invention, when the third transistor is included in the non-pixel region, a plurality of scan lines for transmitting the first scan signal and a second scan signal and a first scan signal and a second scan to the plurality of scan lines. And a scan driver for supplying a signal, a plurality of data lines for transmitting the data signal, and a data driver for supplying a data signal to the plurality of data lines.

In example embodiments, the third transistor may be provided between the scan driver and the pixel region.

In the present invention, the first to third transistors may be NMOS transistors.

In the present invention, the first to third transistors may be formed of oxide TFTs.

In order to solve the above technical problem, the second aspect of the present invention comprises the steps of: a) receiving a first scan signal and a data signal to charge a voltage corresponding to the data signal to a capacitor, b) using the charged voltage A method of driving an organic light emitting display device, the method comprising: emitting an organic light emitting element and c) discharging a charge remaining in the capacitor according to a second scan signal.

In example embodiments, the second scan signal may be a scan signal applied to an adjacent pixel immediately before the first scan signal is applied.

In addition, in the present invention, step c) may ground the capacitor according to the second scan signal.

As described above, when the light emission of the organic light emitting diode is terminated, the second scan signal is applied to the third transistor to remove the voltage stress applied to the gate voltage of the first transistor, which is the driving transistor, thereby deteriorating the first transistor. It is possible to provide a screen with a constant brightness by preventing and preventing the shift of the threshold voltage.

In addition, since the degradation of the first transistor can be prevented, the life of the pixel circuit can be extended.

In addition, since the third transistor is provided in the non-pixel region, the third transistor may have the same aperture ratio as that of the general pixel circuit structure regardless of the light emission method.

In addition, since the third transistor is provided in the non-pixel region, the number of transistors required can be reduced.

1 is a conceptual diagram of an organic EL device.
2 is a circuit diagram of a pixel circuit employed in a conventional organic light emitting display device.
3 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to an exemplary embodiment of the present invention.
4 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to another exemplary embodiment of the present invention.
5 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to another exemplary embodiment of the present invention.
FIG. 6 is a driving waveform diagram for driving the pixel circuit shown in FIGS. 3 to 5.
FIG. 7 is a block diagram illustrating an organic light emitting display device including the pixel circuit of FIG. 3.
FIG. 8 is a block diagram illustrating an organic light emitting display device including the pixel circuit of FIG. 4.

Hereinafter, exemplary embodiments will be described in detail with reference to FIGS. 3 to 8.

3 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to an exemplary embodiment of the present invention. Referring to FIG. 3, an organic light emitting diode OLED, a first transistor Tr ₁ , a second transistor Tr ₂ , a capacitor C, and a third transistor Tr ₃ are included in a pixel circuit of an organic light emitting diode display. Include.

The organic light emitting diode OLED receives light and generates light having any one color among red, green, and blue. The anode of the organic light emitting diode OLED is connected to the second electrode of the first transistor Tr ₁ . In addition, a ground voltage is applied to the cathode of the OLED.

In the first transistor Tr ₁ , a power supply voltage is applied to the first electrode, and a data signal is applied to the gate electrode. In addition, the second electrode of the first transistor Tr ₁ is electrically connected to the organic light emitting diode OLED. The first transistor Tr ₁ applies a current to the organic light emitting diode OLED as a driving transistor.

The data signal is applied to the first electrode of the second transistor Tr ₂ , and the first scan signal is applied to the gate electrode through the first scan line S [n]. In addition, the second electrode of the second transistor Tr ₂ is connected to the gate electrode of the first transistor Tr ₁ . The first scan line S [n] is a scan line for selecting a pixel area to be displayed. The first scanning signal is a scanning signal applied through the first scanning line S [n].

A capacitor (C) is the first and the second terminal is connected to the gate electrode of the transistor (Tr _1), it is a second terminal coupled to the first electrode of the first transistor (Tr _1).

In addition, a first electrode of the third transistor Tr ₃ is connected to the gate electrode of the first transistor Tr ₁ . The ground voltage is applied to the second electrode of the third transistor Tr ₃ , and the second scan signal is applied to the gate electrode through the second scan line S [n-1]. The second scan line S [n-1] is a scan line for applying a scan signal for selecting pixel areas arranged in a row immediately above the pixel area to be displayed. In addition, the second scan signal is a scan signal applied through the second scan line S [n-1].

That is, the second scan signal is a scan signal applied to an adjacent pixel immediately before the first scan signal is applied. That is, when the scan signals are sequentially applied in the row direction in the organic light emitting diode display formed in the form of nxm matrix, if the first scan signal is a scan signal applied to the pixel circuits of the pixels arranged in the nth row, the second scan signal is A scan signal is applied to a pixel circuit of pixels arranged in an n−1 th row that is directly above the pixels to which the first scan signal is applied. When n is 1, that is, in the case of the topmost pixel circuit, the second scan signal may be a scan signal applied to the pixel arranged in the bottom nth row.

Referring to the operation of the driving circuit according to the above-described embodiment, the second transistor Tr ₂ is turned on by the first scan signal, whereby a data signal is applied to the pixel circuit. The capacitor C maintains a data voltage corresponding to the applied data signal for a predetermined period of time. The first transistor Tr ₁ supplies a current corresponding to the voltage applied between both electrodes of the capacitor C to the organic light emitting diode OLED.

Meanwhile, even when the second transistor Tr ₂ is turned off after the data voltage is stored in the capacitor C, a current corresponding to the data voltage stored in the capacitor C is applied to the first transistor Tr _1. The organic light emitting diode OLED is applied to the organic light emitting diode OLED to maintain light emission for a predetermined period of time. Therefore, a positive voltage or a negative voltage is continuously applied to the gate electrode of the first transistor Tr ₁ during the predetermined period.

After the first scan signal is applied and emits light for a predetermined period, a second scan is performed on the gate electrode of the third transistor Tr ₃ before the first scan signal of the next period is applied to the second transistor Tr ₂ . Signal is applied. When the third transistor Tr ₃ is turned on by the second scan signal, the capacitor C is discharged by a ground voltage connected to the second electrode of the third transistor Tr ₃ . That is, all of the charges stored in the capacitor C are released to remove the voltage applied to the gate electrode of the first transistor Tr ₁ .

As mentioned above, the second scan signal is applied to the third transistor Tr ₃ at the end of the light emission of the organic light emitting diode OLED to apply the voltage stress applied to the gate voltage of the first transistor Tr ₁ . Due to the removal, the degradation of the first transistor Tr ₁ , which is the driving transistor, and the shift of the threshold voltage can be prevented, thereby providing a screen having a constant luminance. In addition, since the degradation of the first transistor Tr ₁ may be prevented, the lifetime of the pixel circuit may be increased.

The organic light emitting diode display may include a pixel area including a pixel circuit and a non-pixel area provided outside the pixel area and including a driving circuit for driving the pixel circuit.

In the organic light emitting diode display according to the present exemplary embodiment, the organic light emitting diode OLED, the first transistor Tr ₁ , the second transistor Tr ₂ , the capacitor C, and the third transistor Tr ₃ all have the pixel area. Included in When the third transistor Tr ₃ is included in the pixel area, the organic light emitting diode display may be based on a top emission method. Since the pixel circuit of the organic light emitting diode display according to the present exemplary embodiment includes one transistor in the pixel region than the pixel circuit having the two transistor one capacitor structure, which is a general pixel circuit structure, the aperture ratio of the organic light emitting display may be reduced. Because it can. Therefore, the organic light emitting diode display according to the present exemplary embodiment has a front light emitting structure and displays the screen, thereby preventing deterioration of the first transistor Tr ₁ and at the same time preventing a decrease in the aperture ratio.

4 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to another exemplary embodiment of the present invention. In the embodiment of the pixel circuit of the organic light emitting diode display of FIG. 4, the configuration and the connection relationship of the pixel circuit of FIG. 3 are the same. However, the third transistor Tr ₃ may be included in the non-pixel region.

Since the third transistor Tr ₃ is not included in the pixel region, the organic light emitting diode display according to the present exemplary embodiment may have the same aperture ratio as that of the general two transistor 1 capacitor structure regardless of a light emission method such as top emission or bottom emission. Can have In addition, as in the embodiment of FIG. 3, deterioration of the first transistor Tr ₁ may be prevented by removing the voltage stress applied to the first transistor Tr ₁ using the second scan signal.

5 is a circuit diagram of a pixel circuit of an organic light emitting diode display according to another exemplary embodiment of the present invention. Referring to FIG. 5, similar to FIGS. 3 and 4, the pixel circuit includes an organic light emitting diode, a first transistor Tr ₁ , a second transistor Tr ₂ , a capacitor C, and a third transistor Tr ₃ . do.

In the organic light emitting diode OLED, a power supply voltage is applied to the anode. In addition, the cathode of the organic light emitting diode OLED is electrically connected to the first electrode of the first transistor Tr ₁ .

A first electrode of the capacitor (C) is connected to the gate electrode of the first transistor (Tr _1), a second electrode of the capacitor (C) is connected to a second electrode of the first transistor (Tr _1).

The connection relationship between the first transistor Tr ₁ , the second transistor Tr ₂ , and the third transistor Tr ₃ is the same as the pixel circuit according to the embodiment of FIG. 3 or 4.

The third transistor Tr ₃ is provided outside the pixel region.

In the organic light emitting diode display according to the present exemplary embodiment, similar to FIG. 4, the voltage stress applied to the first transistor Tr ₁ is reduced to prevent deterioration and the life of the pixel circuit is increased. In addition, although one transistor is added to the conventional pixel circuit, the transistor may have the same aperture ratio as the conventional pixel circuit.

3 to 5, the first transistor Tr ₁ , the second transistor Tr ₂ , and the third transistor Tr ₃ may be NMOS transistors. When the first transistor Tr ₁ and the third transistor Tr ₃ are NMOS transistors, the first scan signal and the second transistor Tr ₃ that turn on the first transistor Tr ₁ are turned on. The second scan signal that turns ON is a high signal. The data signal applied to the pixel selected to emit the organic light emitting diode OLED is also a high signal.

The first transistor Tr ₁ , the second transistor Tr ₂ , and the third transistor Tr ₃ may be formed of an oxide thin film transistor (TFT). The production process of oxide TFT OLED panel is simpler than that of conventional silicon, which can greatly reduce the manufacturing cost. If the semiconductor layer is deposited on the oxide instead of the silicon, the equipment investment cost of the existing silicon production line can be greatly reduced, such as the deposition of the thin film directly by sputtering at room temperature without the high temperature deposition process such as PE-CVD (chemical vapor deposition). to be. In addition, the oxide TFT can realize voltage uniformity, which is an advantage of amorphous silicon (a-Si), and high electron mobility, which is an advantage of low temperature polysilicon (LTPS), which is advantageous in improving panel life and high resolution. ZnO or the like may be used as the oxide.

FIG. 6 is a driving waveform diagram for driving the pixel circuit shown in FIGS. 3 to 5. Referring to FIG. 6, voltage changes of the first scan line S [n], the data line D [m], the second scan line S [n-1], and the node A are shown.

In the first period T1, the first scan signal and the data signal are applied to the brazier circuit at the same timing to charge the capacitor with a data voltage corresponding to the data signal.

In the second period T2, a current corresponding to the data voltage is applied to the organic light emitting diode OLED by the first transistor Tr ₁ . The OLED generates light by the applied current.

After the organic light emitting diode OLED emits light for a predetermined time, the second scan signal is first applied to the pixel circuit in the third section T3 before the first scan signal and the data signal of the next period are applied. The charge remaining in the capacitor is discharged in accordance with the second scan signal. That is, when the second scan signal is applied, the third transistor Tr ₃ is turned on so that the first electrode and the second electrode of the third transistor Tr ₃ are turned on, so that the first transistor Tr is turned on. _The ground voltage is applied to the gate electrode of ₁ ).

Node A is a node connected to the gate electrode of the first transistor Tr ₁ . Looking at the voltage change of the node A, the voltage rises to the voltage corresponding to the data signal in the first period T1, and the voltage of the node A decreases slightly during the second period T2. When the second section T2 ends, the ground voltage is applied to the node A in the third section T3 so that the voltage of the node A becomes zero. That is, the voltage stress applied to the gate electrode of the first transistor Tr ₁ is removed.

By repetition of the first section T1 to the third section T3, voltage stress applied to the gate electrode of the first transistor Tr ₁ is eliminated and degradation is prevented.

FIG. 7 is a block diagram illustrating an organic light emitting display device including the pixel circuit of FIG. 3.

Referring to FIG. 7, the organic light emitting diode display 70 according to the present exemplary embodiment includes a scan driver 72, a data driver 73, and an image display 74.

The image display section 74 includes nxm pixel areas 71, n scan lines S [1]… S [n] formed in the row direction, and m data lines D [1]… D [m formed in the column direction. ]), A power supply voltage line (V _DD ) and a ground voltage line (V _SS ).

Each pixel region 71 includes an organic light emitting element and a pixel circuit. The pixel circuit includes a first transistor, a second transistor, a third transistor, and a capacitor. The scan lines S [1]… S [n] transfer the scan signals to the pixel region 71. In addition, the data lines D [1] ... D [m] and the power supply line transfer data signals and power supply voltages to the pixel region 71, respectively.

The scan driver 72 applies a scan signal to the scan lines S [1] ... S [n]. The scan signal is sequentially applied to the scan lines, and the data signal is applied to the pixel circuit in accordance with the scan signals.

The data driver 73 applies a data signal to the data lines D [1]… D [m]. The data signal may be output from a voltage source or a current source in the data driver.

The above-described scan driver 72 and / or data driver 73 may be electrically connected to an image display unit 74 such as a display panel through wire bonding or the like, and may be adhered to and electrically connected to the image display unit 74. The tape may be mounted in a tape carrier package (TCP) or the like in the form of a chip. In addition, the scan driver 72 and / or the data driver 73 may be mounted in the form of a chip in a flexible printed circuit (FPC) or a film that is adhered to and electrically connected to the image display 74. This structure is commonly referred to as a chip on film (COF) method. In addition, the scan driver 72 and / or the data driver 73 may be mounted directly on the glass substrate of the image display portion 74 and in a drive circuit formed on the glass substrate in the same layers as the scan line, data line and TFT. It may be mounted.

The power supply voltage line V _DD and the ground voltage line V _SS may be provided to the pixel regions 71 of the image display unit 74 collectively by the common electrode. Alternatively, a separate wiring may be formed to be provided in the pixel areas 71.

In addition, in the scan lines illustrated in FIG. 7, the scan line S [n] formed at the top may be the same line as the scan line S [n] formed at the bottom. Alternatively, a new line to which the same scan signal as the scan line S [n] formed at the bottom is applied may be separately formed on the scan line S [1] to apply the scan signal to the pixel area arranged in the first row. .

FIG. 8 is a block diagram illustrating an organic light emitting display device including the pixel circuit of FIG. 4.

Referring to FIG. 8, similar to FIG. 7, the organic light emitting diode display 80 includes a scan driver 82, a data driver 83, and an image display 84.

Meanwhile, the plurality of pixel areas 81 included in the image display unit 84 do not include the third transistor Tr ₃ . As shown in FIG. 8, the third transistors Tr ₃ may be provided between the scan driver 82 and the pixel region 81.

As shown in FIG. 8, when the third transistor Tr ₃ is provided between the scan driver 82 and the pixel region 81, the number of transistors may be reduced. If included in the inner third transistor (Tr ₃₎ the pixel region as shown in 7, it shall therefore be provided with a third transistor (Tr ₃₎ in each pixel region to form a total of nxm transistors. However, when the pixel regions 81 of the same row to which the same scan signal is applied share the same third transistor Tr _{3 as} shown in FIG. 8, only n transistors, which are the number of rows, are needed.

As illustrated in FIG. 7, the scan line S [n] connected to the third transistor Tr ₃ provided at the top may display the same scan line as the scan line provided at the bottom. Alternatively, a separate scan line or the same scan signal may be applied to the bottom scan line.

As described above, the organic light emitting diode display according to the present embodiment not only prevents deterioration of the first transistor, which is a driving transistor, but also reduces the number of transistors required by providing the third transistor Tr ₃ in the non-pixel region. You can do it.

The above detailed description and drawings are merely exemplary of the present invention, but are used only for the purpose of illustrating the present invention and are not intended to limit the scope of the present invention as defined in the meaning or claims. Therefore, those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical protection scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (13)

Turning on the second transistor by the first scan signal to receive a data signal, and charging a capacitor with a voltage corresponding to the data signal;
Illuminating the organic light emitting device by applying a current corresponding to the charged voltage to the organic light emitting device through a first transistor; And
And turning on the third transistor by the second scan signal to apply a ground voltage to one terminal of the capacitor to discharge the charge remaining in the capacitor. The method of claim 1,
And the second scan signal is a scan signal applied to an adjacent pixel immediately before the first scan signal is applied. The method of claim 2,
And the adjacent pixels are pixels of a previous row. The method of claim 1,
And the first to third transistors are NMOS transistors. The method of claim 1,
And the first to third transistors are oxide TFTs. Organic light emitting device;
A first transistor having a first electrode to which a power voltage is applied, a second electrode electrically connected to the organic light emitting element, and a gate electrode;
A second transistor having a first electrode to which a data signal is applied, a second electrode connected to the gate electrode of the first transistor, and a gate electrode to which a first scan signal is applied;
A capacitor having a first terminal connected to the gate electrode of the first transistor and a second terminal connected to the first electrode of the first transistor; And
A first electrode connected to the gate electrode of the first transistor, a second electrode to which a ground voltage is applied, and a gate electrode to which a second scan signal is applied, and when turned on by the second scan signal, And a third transistor configured to discharge the charge remaining in the capacitor by applying the ground voltage to the first terminal. The method of claim 6,
And the second scan signal is a scan signal applied to an adjacent pixel immediately before the first scan signal is applied. The method of claim 7, wherein
And the adjacent pixels are pixels of a previous row. The method of claim 6,
The organic light emitting diode display may include a pixel area including a pixel circuit; And
And a non-pixel area disposed outside the pixel area and including a driving circuit for driving the pixel circuit. The organic light emitting diode display of claim 9, wherein the third transistor is included in the pixel area. The method of claim 10,
The organic light emitting diode display may have a top light emitting structure. The method of claim 6,
The first to third transistors are NMOS transistors. The method of claim 6,
The first to third transistors are oxide TFTs.

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